Do browsers support 226 IM Used?
No mainstream browser requests delta encoding, so a browser will never see a 226 in normal browsing. It is used only by specialised clients that opt in with A-IM.
HTTP 226 IM USED
The response is a delta encoding applied to the current instance. 226 belongs to the 2xx success class and is defined in RFC 3229.
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About HTTP 226
What Does HTTP 226 IM Used Mean?
The response is a delta encoding applied to the current instance. The code is registered with IANA and defined in RFC 3229.
Success: the request was received, understood, and accepted. Every code from 200 to 299 shares that general meaning, so a client that does not recognise 226 must treat it like 200.
When Do You See a 226 Response?
226 IM Used belongs to delta encoding for HTTP (RFC 3229). A client that already holds an old version of a resource sends A-IM naming the delta formats it understands, and the server answers 226 with only the differences rather than the full document. The idea was to save bandwidth on frequently changing feeds and pages. Adoption stayed very low, so it shows up mostly in experiments and a few feed servers.
How Do You Handle a 226?
Clients that did not send A-IM should never receive a 226; if one arrives, treat the body as unusable without the base version. A server sending it must include an IM header naming the instance manipulation applied, plus a strong ETag so the client can track versions. For new designs, ordinary compression with gzip, Brotli, or zstd plus conditional requests with 304 usually captures most of the savings.
What Does a 226 Response Look Like?
HTTP/1.1 226 IM Used
IM: vcdiff
ETag: "v9"
Delta-Base: "v8"226 is not cacheable by default: a browser or CDN stores it only when Cache-Control or Expires explicitly allows it.
To inspect a real one without sending it anywhere:
- Copy the response headers from your browser Network tab, or from
curl -i. - Paste them into the Header decoder to see every header explained.
- Check caching in the Cache lane.
Which Status Codes Are Related to 226?
- 200 OK: The request succeeded. The body depends on the method.
- 304 Not Modified: The cached copy is still valid, so no body is sent. Answer to If-None-Match or If-Modified-Since.
- 206 Partial Content: Only the requested byte range is returned. Used for resumable downloads and video seeking.
Browse every code on the HTTP status codes page.
What Other Lanes Are in the Reference Section?
- HTTP status codes: Every IANA-registered status code, with a page per code.
- MIME types: File extensions to Content-Type, searchable both ways, plus a Content-Type parser.
- HTTP methods: GET, POST, PUT, PATCH, DELETE and the rest: safe, idempotent, cacheable.
Elsewhere in 2http: Header decoder, Cookie decoder, Cache-Control decoder, CORS checker, User-agent parser, and all eleven lanes on the 2http home page. Part of the 2suite.io privacy-first ecosystem.
Frequently Asked Questions
What is the IM header on a 226?
It names the instance manipulation the server applied, such as a delta algorithm or compression, so the client knows how to rebuild the full resource from its stored base copy.
Is HTTP 226 an error?
No. 226 is a success response, not an error.
Is a 226 response cacheable by default?
No. Caches store a 226 only when Cache-Control or Expires explicitly allows it.
How Can You Verify That Nothing Leaves This Browser?
You do not have to take our word that your headers stay here. Developer tools show every request a page makes, so you can watch this page work and confirm that nothing is sent. The check takes under a minute.
- Open your browser developer tools with
F12, orCmd+Option+Ion a Mac, and select the Network tab. - Reload the page and let it finish. You will see the page's own files, its web fonts, and one anonymous page-view request. Nothing has been typed yet, so none of them can contain your headers.
- Click the clear icon in the Network tab so that the log is empty.
- Paste real headers into any lane and read the result.
- Look at the Network tab again. It is still empty. For extra proof, turn on offline mode after the page loads and every lane keeps working.
Only two things can add a line to that log, and neither carries your headers. Submitting the feedback form sends the vote and comment you typed. Typing characters in a script the page has not displayed yet, such as Cyrillic or Greek, can make the browser fetch the matching font file, and that request names only the font.
2http never sends a request on your behalf: the request builder writes code for you to run, and the meta inspector reads source you paste instead of fetching a URL. The production site also sends a Content-Security-Policy header that tells your browser to refuse connections to anywhere except this site and the anonymous analytics endpoint.